1. Field
The present disclosure relates generally to methods and apparatus for acquiring timing information in wireless communication systems, and more specifically to methods and apparatus for doing so in multiple-input, multiple-output (MIMO) systems, especially those employing orthogonal frequency division multiplexing (OFDM) to permit multiple-access.
2. Background
Wireless communication systems including those defined by the IEEE 802.11 series of specifications transmit information in the form of symbols represented by signals having very specific characteristics of frequency, phase, and consequently, timing. In the 802.11n standard, for example, the timing by which information may be derived from a transmitted frame is defined relative to an origin. That origin is defined by two training fields that precede the data-carrying portion of the frame.
Conventionally, the origin is identified by performing autocorrelation on a received signal, and identifying a peak in the resulting autocorrelation function. In MIMO systems, as well as other transmission systems, signals transmitted from a transmitter to a receiver may take paths that reduce their coherence, introducing errors referred to as multi-path errors. For example, part of a signal may travel directly from the transmitter to the receiver, while another part of the signal may reflect off of nearby objects on the way from the transmitter to the receiver. As a result, the resolution with which the origin can be determined may be limited due to smearing of the peak of the autocorrelation function so that the time at which the peak occurs cannot be precisely determined. There is therefore a need in the art for methods and apparatus that more accurately determine the time of the origin.